In semiconductor manufacture, extremely small electronic devices are formed in separate dies in a thin, flat semiconductor wafer. In general, various materials which are either conductive, insulating, or semiconducting are utilized in the fabrication of integrated circuitry on semiconductor wafers. These materials are patterned, doped with impurities, or deposited in layers by various processes to form integrated circuits.
Increasing circuitry miniaturization and a corresponding increase in density has resulted in a high degree of varying topography being created on an outer wafer surface during fabrication. It is often necessary to polish a wafer surface having varying topography to provide a substantially planar surface. One such process is chemical-mechanical polishing. In general, this process involves holding and rotating a thin, flat wafer of the semiconductor material against a wetted polishing surface under controlled chemical, pressure, and temperature conditions. A chemical slurry containing a polishing agent, such as alumina or silica, is used as the abrasive medium. Additionally, the chemical slurry contains selected chemicals which etch various surfaces of the wafer during processing. The polishing effect on the wafer results in both a chemical and mechanical action.
A particular problem encountered in chemical-mechanical polishing is the determination that the surface has been planarized to a desired end point. It is often desirable, for example, to remove a thickness of oxide material which has been deposited onto a substrate, and on which a variety of integrated circuit devices have been formed. In removing or planarizing this oxide, it is desirable to remove the oxide to the top of the various integrated circuits devices without removing any portion of the devices. Typically, this planarization process is accomplished by control of the rotational speed, downward pressure, chemical slurry, and time of polishing.
The planar endpoint of a planarized surface is typically determined by removing the semiconductor wafer from the planarization apparatus and physically measuring the semiconductor wafer by techniques which ascertain dimensional and planar characteristics. If the semiconductor wafer does not meet specification, it must be loaded back into the planarization apparatus and further processed. Alternately, an excess of material may have been removed from the semiconductor wafer, rendering the part as substandard.
Certain techniques have been developed for in situ detection of chemical-mechanical planarization. Such are disclosed, by way of example, in our U.S. Pat. Nos. 5,036,015; 5,069,002; and 5,081,796. One such technique employs laser interferometry in situ to determine CMP end point. Such a technique employs maximizing of internal reflection or absorption of laser light into a light transmissive layer of material. The intensity of remaining light emanating outwardly through or from the upper surface is monitored. Thickness and planarity are determinable therefrom. Such is described in more detail with respect to FIGS. 1 and 2.
FIG. 1 diagrammatically illustrates a semiconductor wafer 10 comprised of a bulk substrate 12 and an overlying, unpolished upper silicon dioxide layer 14 having uneven topography. Incident laser light rays indicated by numerals 16a, 16b, 16c and 16d impinge and refract through upper layer 14, and bounce off of substrate layer 12 and outwardly of layer 14 as reflected light 20. As is apparent, reflected light 20 is modulated (changed) in comparison to the incident light 16.
Laser interferometry functions by seeking to attain substantial subsequent internal reflection of the laser light reflecting off of the substrate from the upper surface of the overlying layer and back onto the lower base substrate. Such phenomena is maximized when the thickness of the layer through which the incident laser light passes is approximately equal to an integer multiple of the wave length of the incident light. It is here where internal reflection, or absorption, of the light by the overlying layer is maximized. For example, assume the incident laser light rays 16a, 16b, 16c and 16d in FIG. 1 have an integer multiple wave length equal to the illustrated optical path within the least thick portions of overlying layer 14. As shown, incident rays 16a and 16c impinge on outer layer 14 in locations such that the first reflection off of the upper surface of substrate 12 results in additional reflection downward onto and off substrate 12 before escape from layer 14. On the other hand, incident rays 16b and 16d upon reflection off of substrate layer 12 reflect into thicker portions of layer 14 and immediately outwardly from such portions. Accordingly, collectively the incident light is modulated and useful information is determinable therefrom.
As the wafer becomes more planar, less light and lower intensity light is emitted from the outer layer. FIG. 2 illustrates the effect as planarity increases on wafer 10a. As is apparent, essentially all of the incident impinging light is internally reflected or absorbed. Accordingly, measurement of intensity as a function of time enables the operator to determine achievement of a desired thickness of layer 14a and correspondingly when substantial planarity is achieved.
Laser interferometry is not without some inherent drawbacks. First, such measures absolute intensity of light emitting from overlying substrate layer 14. Such would be impacted by the inherent degree of translusivity of the material of layer 14, and thus varies dependent upon the material 14 being polished. Second and correspondingly, laser interferometry is incapable of being utilized on opaque layers. Third, the fact that internal reflection or absorption of light occurs at any integer multiple of the incident wave length adds complexity. Specifically, the operator cannot directly tell whether the thickness being measured by the incident light is actually the desired finished thickness or some integer multiple thereof. To overcome such anomaly, the prior art utilizes multiple light wave lengths. From such collective information, both thickness and substantial planarity of a polished layer is determinable. Such does, however, require multiple light sources and further complexity in interpreting the detected light emanating from the substrate.
It would be desirable to develop improved methods of end point detection in chemical-mechanical polishing.